The dimensions are exactly the same.Except the iPhone 7 camera is larger.

1. Speaker Placement:The iPhone 7 and 7 Plus have stereo speakers, one on the top and one on the bottom of the phone. If you iPhone 6/6S case is open at the top, you may be OK here, but if not, the sound will likely be muffled.

2. Size of Camera Opening: The camera and flash combos on the iPhone 7 and 7 Plus are larger than on the iPhone 6 and 6 Plus phones. So, the size of the camera opening may be a problem. This is particularly true of the iPhone 7 Plus, which has a much larger dual camera system as compared to the iPhone 6/6S Plus.

3. Headphone Jack and Bottom Openings: The big news for about a year has been that Apple is eliminating the headphone jack on the iPhone 7 and iPhone 7 Plus. The jack is gone, for cases that are not totally open on the bottom, things will not align correctly. Where the headphone jack on the iPhone 6/Plus was, there is now a microphone. The shape is different. The mic may work, but it will not line up nicely with the old jack opening. If your case is totally open at the bottom, this shouldn't be a problem.

Our conclusion. Except for the size of the camera & flash combo opening, you will likely be able to get by with your iPhone 6/6S 6/6S Plus case...temporarily. But there will likely be a number of problems that will degrade your enjoyment of your new iPhone 7/7 Plus case, that will lead to a new purchase.